Why doesn't DWTS just play the original songs?

Almost certainly, yes.

I was just watching it and thinking the EXACT same thing. That Queen song just really cut me up. Cracking? How are you a professional singer on a hit television show and doing that?

AGREE. And it’s definitely a royalty thing.

You answered your own question. :)

Here’s the thing, though: wouldn’t it be cheaper to pay a onetime royalty instead of hiring a full band, having (crappy) singers, and paying the onetime royalty on top?

Here’s another thing – why not just have good singers?

I assume none of you are competitive dancers. The piece has to be cut and worked to the time limits allowed on the show (and of the competitive rules) and you’ll notice the most danceable parts of a song are not necessarily together and of course, no one wants the chorus of most songs repeated. If they had to work out a routine for a full song, every song would have to be the same length as to remove anyone having an advantage. Finally, performance ballroom is (obviously) exhausting at 1½ to 2 minutes much less the full 4 minutes of a performance original song.